AN A320 PLT RPTS RESPONDING TO A CLB TCAS RA FOR A NON RPTED C152 WHILE LEVEL AT 3000 FT TALKING WITH SOCAL ON THE ONT PRADO 6 SID.

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ON INITIAL DEP FROM ONT ON THE PRADO 6 SID WE WERE INSTRUCTED BY ONT TWR TO MAINTAIN 3000 FT UNTIL FURTHER ADVISED. LEVELING AT 3000 FT AND TALKING NOW TO SOCAL DEP CTL; WE WERE INSTRUCTED TO LOOK FOR VFR TFC AT THE 12 O'CLOCK POS; 3500 FT; PASSING L TO R. WHILE LOOKING FOR THAT TFC; WE NOTICED WHAT LOOKED TO BE A C152 AT ABOUT OUR 12:30 O'CLOCK POS; LOW (HE WAS 500 FT BELOW US PER THE TCAS READOUT); TRAVELING FROM L TO R. TCAS INDICATED THAT THE C152 WAS CLBING BUT HAD NOT YET ISSUED EITHER A TA OR AN RA. WANTING TO KEEP A VISUAL ON THAT ACFT AS IT STARTED DISAPPEARING UNDER THE NOSE; I MADE A L TURN TO PASS BEHIND THE C152 AND MAINTAIN VISUAL CONTACT WITH HIM. AS I WAS MAKING THE SLIGHT TURN; TCAS ISSUED A CLB ADVISORY WHICH I COMPLIED WITH. WE PASSED BEHIND AND ABOVE SUBJECT TFC. SOCAL ASKED IF WE HAD TURNED OFF COURSE TO AVOID THE TFC AND WE RESPONDED IN THE AFFIRMATIVE. SHE THEN ASKED IF WE HAD RECEIVED AN RA FROM TCAS. WE AGAIN ANSWERED IN THE AFFIRMATIVE. SHE INDICATED TO US THAT SHE HAD NOT BEEN TALKING TO THE C152. THE INITIAL VFR ACFT AT 3500 FT WAS NO LONGER A FACTOR AND WE WERE CLRED TO CLB TO A HIGHER ALT AND RESUME OUR OWN NAV VIA THE PRADO 6 DEP. OBVIOUSLY; THIS COULD HAVE BEEN PREVENTED HAD THE C152 BEEN TALKING TO SOCAL AND WORKING WITHIN THE SYS WHICH; APPARENTLY; HE WASN'T.